The individual forms of cytochrome P-450 display unique substrate specificity and reactivity profiles toward a variety of drugs and carcinogens. Differences in cytochrome P-450 phenotype may relate to individual differences in sensitivity to certain drugs and susceptibility to carcinogenesis. Monoclonal antibodies (MAbs) to cytochromes P-450 have been used as specific probes for the cytochromes P-450 in human liver. Western blot analysis with antisera to rat ethanol-induced P-450, a form with high nitrosamine metabolizing activity, detected a P- 450 in human liver homogenates. Individual variation in the level of this P-450 was observed. MAb 1-98-1 to this rat P-450 also detected a related human P-450 in liver microsomes by Western blot analysis. A radioimmunoassay for P-450 in human liver microsomes was developed using this MAb. Further refinement will provide a rapid, efficient method for screening large numbers of samples from human tissues.